1. Field of the Invention
The present invention relates to a plasma processing apparatus for manufacturing semiconductor devices.
2. Related Background Art
Conventional techniques of radiating a microwave from a microwave antenna and irradiating processing gas with it in order to generate plasma thereof and cause the generated plasma to act on a Si substrate for manufacturing semiconductor devices for a predetermined processing operation include one that utilizes a microwave waveguide provided with one or more than one slits and one adapted to feed a microwave to a disk provided with coaxially arranged slits (see Patent Document 1: Japanese Patent Application Laid-Open Publication No. 9-129613 and Patent Document 2: Japanese Patent Application Laid-Open Publication No. 11-111620).
FIG. 1 of the accompanying drawings illustrates a conventional microwave antenna that is described in Patent Document 1. The microwave antenna comprises a waveguide A that has a bottom B and a reflection surface C. A pair of slits D are cut through the bottom B of waveguide A so as to show a profile with a width that is narrowed stepwise toward the reflection plane C of the waveguide A in order to allow the reflected wave reflected by the reflection surface C and the incident wave to produce a uniformly synthesized wave.
FIG. 2 of the accompanying drawings illustrates another conventional microwave antenna that is described in Patent Document 2. It is a radial line slot antenna. The radial line slot antenna is provided with a number of coaxially arranged slots E and the circularly polarized microwave generated from the slots E is introduced into a vacuum chamber so as to generate and excite plasma by means of the electric field of the microwave.
The inventors of the present invention actually prepared antennas according to the descriptions of Patent Documents 1 and 2 and operated them to see how they perform in an experiment As a result of the experiment, it was found that the both antennas can be driven to operate only within a narrow pressure range and a narrow applicable electric power range for normal electric discharges as shown in Tables 1 and 2 below. Tables 1 and 2 summarily show the results of the experiments obtained respectively by using a microwave antenna according to Patent Document 1 and a microwave antenna according to Patent Document 2.
TABLE 1applied electricpower/pressure20 Pa50 Pa90 Pa1.0 kWXX◯1.5 kW◯◯X1.8 kW◯XX2.0 kWXXXN.B.:◯ indicates that a normal electric discharge was allowed.X indicates that a normal electric discharge was not allowed because of a large reflected wave.
TABLE 2applied electricpower/pressure20 Pa50 Pa90 Pa1.0 kWXX◯1.5 kWXX◯1.8 kWXX◯2.0 kWXX◯N.B.:◯ indicates that a normal electric discharge was allowed.X indicates that a normal electric discharge was not allowed because of a large reflected wave.
The reason for the narrow pressure range and the narrow applicable electric power range may be that a large voltage standing wave ratio (VSWR) of the reflected wave/the progressive wave arises due to an unbalanced electromagnetic field of microwave that is found right below the microwave introducing window. Therefore, the electromagnetic field that is found right below the microwave introducing window below a microwave antenna as described in each of Patent Documents 1 and 2 was determined by simulation. As a result, local concentrations and asymmetries of electromagnetic field were found as shown in FIGS. 3A, 3B, 3C and 3D and FIGS. 4A, 4B, 4C and 4D.